In the drilling of wells, particularly for oil and gas, holes are drilled deep into the earth and tubular strings of pipe are inserted within the resulting wellbore. One typical operation of this type involves inserting a tubular string of casing in the well and cementing the casing in the wellbore. That operation involves forcing cement grout into the annulus between the wall of the wellbore and the casing. The objective in such a cementing operation is to obtain a circumferentially continuous distribution of the cement in the annulus so that when the cement sets up it will preclude formation or producing fluids from migrating vertically between the casing and the wellbore wall. Due to the fact that the casing is generally never totally centered within the wellbore there are areas within the wellbore in which the cement often cannot flow in a uniform manner. This often leads to incomplete surrounding of the casing with cement which in turn leads to undesirable leakage of formation or producing fluids.
When a tubular string is rotated in a conventional manner every point aroung the circumference of the tubular string is in the large standoff area for a time. This improves the cement bond to the tubular string but does not improve the cement bonding to the well bore.
In the past one of the techniques used to try to insure that cement is applied in a circumferentially continuous manner around the casing has been to rotate the casing while moving it up and down axially within the wellbore as the cement is being pumped into the annulus. World Oil, "Cementing Oil and Gas Wells", pages 32-37 by G.O. Suman, Jr and R.C. Ellis (1972) discloses the use of rotation or rotation and reciprocation to provide additional displacement forces which aid in distributing the cement.
The reciprocation of casing is not, however, universally practiced due to the risk that the casing may become stuck above the desired location depth. Further that technique is often resisted because of the risk that the casing string may be broken.
U.S. Pat. No. 4,312,405 discloses a variation of the technique in which a stab-in string within the tubular casing string is attached to the lower end of the tubular casing string and is reciprocated while the top of the tubular casing string is maintained at stationary level. The patent states that when the inner pipe is moved axially in that manner it causes buckling of the lower end of the casing to change the standoff of the casing.
An object of the present invention is to provide method and apparatus useful for causing the area of largest standoff between a tubular string and the wellbore to be moved radially around within in a zone in the wellbore without having to employ any attached device which is moved axially in the well.
Another object of the present invention is to provide method and apparatus useful for causing the area of largest standoff between a tubular string and the wellbore to be moved circumferentially in the wellbore, thus making not just a few points, but every point along the circumference of the wellbore the large standoff area at some time. Furthermore, not just one depth zone but several can be selected for this special treatment.
Other aspects, objects, and advantages of the present invention will be apparent from the following disclosure taken in conjunction with the accompanying Figures.